This invention relates to the measurement of defects in the construction of flexible web material, and more particularly to the measurement of defects in greige (pronounced xe2x80x98grayxe2x80x99) goods; fabrics in their raw state, typically, right from the loom.
Many products are manufactured most efficiently in continuous processes that produce long sections of flat material in which the length and width are much larger than the thickness. Continuous webs of materials, including plastic sheet, paper, floor coverings, tire plies and textiles, are typically made into rolls and cut to length after the bulk of the manufacturing processes are performed. Typically, a series of value-adding operations such as drying, pressing, tinting, curing, printing and coating are performed at multiple processing stations.
Because each processing stage adds value, the work-in-progress increases in value as it moves down the production line. It is therefore desirable to inspect the output of each stage both to eliminate adding downstream value to unrepairable product and to provide feedback control to the previous stage. For example, if greige goods at the output of a loom are detected as having a large section with missing threads, the problem may be corrected before a significant additional amount of faulty cloth is woven.
The greige goods to be inspected are woven threads in sheets of various widths, some about 70xe2x80x3 wide. As shown in FIG. 7, two axes of threads are interlaced together at right angles; the warp threads W run the length of the goods, the pick threads P are woven above and below adjacent warp threads across the goods. The woven cloths have a specified number of ends per inch, generally in the range of 40 to 200 epi. The threads vary in diameter, but are often about 10 mils, and are comprised of even smaller fibers spun together. These fibers are either natural fibers, like cotton, or synthetics like polyester. Both natural and synthetic fibers are dielectrics with relative permittivities greater than 1. The spun fibers enclose some air so that the resulting thread is made up of a combination of fiber particles and air, resulting in the thread having an effective relative permittivity of only a little more than 1.
In a production setting, greige goods move in the direction of the warp threads at speeds on the order of 300 to 1200 picks per minute.
Capacitance sensing is a promising technique for measuring parameters on sheet or web materials. U.S. Pat. No. 5,281,921 of J. Novak et al. describes a system where various parameters of sheet material, such as edge smoothness and weld thickness, are measured by the change in capacitance between two spaced electrodes in proximity to the system under test. U.S. Pat. No. 5,537,048 of J. Novak described a roller apparatus for measuring the impedance of web materials containing conductive wires that passed over the roller.
Capacitance sensors can measure either the offset of the sensor from a conductive workpiece or the permittivity of a dielectric workpiece that is in contact with the sensor. Great difficulties arise when attempting other types of measurements with capacitance sensors. They are not suited for inspecting the interior of metals or for looking through metals. If used to inspect dielectric materials that are not in contact with the sensor, a change in offset will produce the same type of sensor response as a change in dielectric constant. Thus, strict offset control is necessary to ensure reliable data.
Any pair of conductors forms a capacitor, and that capacitance is the ratio of the charge to the voltage across the conductors. The capacitance of a pair of conductors is determined by their geometry and by all the media surrounding them. Any variation in the surrounding media (such as variations in the thread thickness and distribution for greige goods) will cause a variation in the measured capacitance. In general, capacitances are either measured or are calculated from computer boundary value or finite element models, as simpler mathematical models provide only estimates of actual capacitances since they generally ignore fringing fields.
FIG. 1 shows the two conductors 2, 4 of a capacitor to be metal plates, of length L, height H, and width W. Two common arrangements of capacitor plates that are easily realizable using printed wiring board technology are aligned parallel plates or coplanar plates, although a variety of other arrangements exist. For any such capacitor, the capacitance is the sum of the direct (Cp) and fringing field contributions (Cf).
The direct contribution for the aligned parallel plate case, assuming a uniform medium between the plates with dielectric constant xcex5, is approximated to be                               C          p                =                  ϵ          ⁢                                    L              ⁢                              xe2x80x83                            ⁢              H                        G                                              (        1        )            
where G is the gap or separation. This contribution is due to all the field lines starting on one plate and ending on the other that are constrained to the volume immediately between the two plates and are perpendicular to the plates. The fringing field contribution is due to all the field lines that are not constrained to the volume immediately between the plates. If the value of H is large compared to G, then the fringing contribution is small compared to the direct contribution. For such a case, the height of the plates and the dielectric constant outside the plates plays a very small role in determining the total capacitance.
Such an arrangement (parallel plates) would make a useful sensor only if the material 9 to be inspected were placed between the plates and acted as the dielectric of the capacitor. This setup can be called a two-sided measurement because electronic components must be placed on both sides of the material. A two-sided measurement can be very sensitive but also requires a complicated mechanical mount.
Alternatively, the coplanar plate arrangement (with parallel lengths L) can be used to make a one-sided measurement of a material 8 that is a fixed distance above the plates. For this case, the capacitance is again the sum of the direct and fringing field contributions. Here, however, the direct contribution is made purposely small since H less than  less than G making Cp approach zero. Thus, for this geometry, the fringing field capacitance becomes the dominant term. For a printed wiring board, metal thicknesses are usually on the order a few mils but the fringing field contribution remains about as large as it was in the parallel plate case. Thus, it now contributes a larger share of the total capacitance resulting in a sensor that is very sensitive to changes in the fringing field.
The fringing field contribution can be treated as two parts: the first part is above the plates and the second part is below the plates. If the plates are conductors on the top side of a circuit board, then the second part can be contained inside the circuit board by proper grounding of a metallization layer covering the other side of the circuit board, and its contribution can be controlled and kept constant. The first part is determined by the dielectric material 8 above the plates and by any conductive material in close proximity. By placing a dielectric material in contact with the plates or at a fixed offset, and by removing any conductive material close to the plates, the change in capacitance between the plates will be almost entirely determined by the change in dielectric constant of the dielectric material.
If conductive material is placed in close proximity to the plates, then either of two effects will occur depending on whether the conductive material is ac-grounded or not. If the material is ungrounded, then field lines from the driving plate will excite the material, which will reradiate and enhance the capacitance between the plates. If the material is grounded, then field lines from the driving plate will be shunted to ground, and the capacitance between the plates is reduced. Similarly, use of a ground plane on the printed circuit board layer immediately below the plates will minimize the backside capacitance.
It is an object of this invention to provide a coplanar electrode arrangement on a flexible circuit board that is curved to conform to the path of the material.
It is another object of the invention to provide a system for detecting defects in a web of material passing over coplanar sensors.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention may comprise a system for electrically measuring variations over a flexible web comprising a web support structure; a flexible web affixed to the web support structure and extending under longitudinal tension along a path extending from a first end to a second end; and a capacitive sensor comprising a dielectric support having rigidly mounted thereon spaced, electrically conductive, transmit and receive electrodes. A sensor support rigidly holds a portion of the dielectric support against said web with sufficient force to cause the path to be longer than the shortest distance between the first and second ends, the electrodes being adjacent the dielectric support. The web moves relative to the sensor.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.